package Exception::Class;

use 5.008001;

use strict;

use Exception::Class::Base;
use Scalar::Util qw(blessed);

our $BASE_EXC_CLASS;
BEGIN { $BASE_EXC_CLASS ||= 'Exception::Class::Base'; }

our $VERSION = '1.29';

our %CLASSES;

sub import
{
    my $class = shift;

    local $Exception::Class::Caller = caller();

    my %c;

    my %needs_parent;
    while (my $subclass = shift)
    {
        my $def = ref $_[0] ? shift : {};
        $def->{isa} = $def->{isa} ? ( ref $def->{isa} ? $def->{isa} : [$def->{isa}] ) : [];

        $c{$subclass} = $def;
    }

    # We need to sort by length because if we check for keys in the
    # Foo::Bar:: stash, this creates a "Bar::" key in the Foo:: stash!
 MAKE_CLASSES:
    foreach my $subclass ( sort { length $a <=> length $b } keys %c )
    {
        my $def = $c{$subclass};

        # We already made this one.
        next if $CLASSES{$subclass};

        {
            no strict 'refs';
            foreach my $parent (@{ $def->{isa} })
            {
                unless ( keys %{"$parent\::"} )
                {
                    $needs_parent{$subclass} = { parents => $def->{isa},
                                                 def => $def };
                    next MAKE_CLASSES;
                }
            }
        }

        $class->_make_subclass( subclass => $subclass,
                                def => $def || {},
                              );
    }

    foreach my $subclass (keys %needs_parent)
    {
        # This will be used to spot circular references.
        my %seen;
        $class->_make_parents( \%needs_parent, $subclass, \%seen );
    }
}

sub _make_parents
{
    my $class = shift;
    my $needs = shift;
    my $subclass = shift;
    my $seen = shift;
    my $child = shift; # Just for error messages.

    no strict 'refs';

    # What if someone makes a typo in specifying their 'isa' param?
    # This should catch it.  Either it's been made because it didn't
    # have missing parents OR it's in our hash as needing a parent.
    # If neither of these is true then the _only_ place it is
    # mentioned is in the 'isa' param for some other class, which is
    # not a good enough reason to make a new class.
    die "Class $subclass appears to be a typo as it is only specified in the 'isa' param for $child\n"
        unless exists $needs->{$subclass} || $CLASSES{$subclass} || keys %{"$subclass\::"};

    foreach my $c ( @{ $needs->{$subclass}{parents} } )
    {
        # It's been made
        next if $CLASSES{$c} || keys %{"$c\::"};

        die "There appears to be some circularity involving $subclass\n"
            if $seen->{$subclass};

        $seen->{$subclass} = 1;

        $class->_make_parents( $needs, $c, $seen, $subclass );
    }

    return if $CLASSES{$subclass} || keys %{"$subclass\::"};

    $class->_make_subclass( subclass => $subclass,
                            def => $needs->{$subclass}{def} );
}

sub _make_subclass
{
    my $class = shift;
    my %p = @_;

    my $subclass = $p{subclass};
    my $def = $p{def};

    my $isa;
    if ($def->{isa})
    {
        $isa = ref $def->{isa} ? join ' ', @{ $def->{isa} } : $def->{isa};
    }
    $isa ||= $BASE_EXC_CLASS;

    my $version_name = 'VERSION';

    my $code = <<"EOPERL";
package $subclass;

use base qw($isa);

our \$$version_name = '1.1';

1;

EOPERL


    if ($def->{description})
    {
        (my $desc = $def->{description}) =~ s/([\\\'])/\\$1/g;
        $code .= <<"EOPERL";
sub description
{
    return '$desc';
}
EOPERL
    }

    my @fields;
    if ( my $fields = $def->{fields} )
    {
        @fields = UNIVERSAL::isa($fields, 'ARRAY') ? @$fields : $fields;

        $code .=
            "sub Fields { return (\$_[0]->SUPER::Fields, " .
            join(", ", map { "'$_'" } @fields) . ") }\n\n";

        foreach my $field (@fields)
        {
            $code .= sprintf("sub %s { \$_[0]->{%s} }\n", $field, $field);
        }
    }

    if ( my $alias = $def->{alias} )
    {
        die "Cannot make alias without caller"
            unless defined $Exception::Class::Caller;

        no strict 'refs';
        *{"$Exception::Class::Caller\::$alias"} = sub { $subclass->throw(@_) };
    }

    eval $code;

    die $@ if $@;

    $CLASSES{$subclass} = 1;
}

sub caught
{
    my $e = $@;

    return $e unless $_[1];

    return unless blessed($e) && $e->isa( $_[1] );
    return $e;
}

sub Classes { sort keys %Exception::Class::CLASSES }

1;

__END__

=head1 NAME

Exception::Class - A module that allows you to declare real exception classes in Perl

=head1 SYNOPSIS

  use Exception::Class
      ( 'MyException',

        'AnotherException' =>
        { isa => 'MyException' },

        'YetAnotherException' =>
        { isa => 'AnotherException',
          description => 'These exceptions are related to IPC' },

        'ExceptionWithFields' =>
        { isa => 'YetAnotherException',
          fields => [ 'grandiosity', 'quixotic' ],
          alias => 'throw_fields',
        },
      );

  # try
  eval { MyException->throw( error => 'I feel funny.' ) };

  my $e;
  # catch
  if ( $e = Exception::Class->caught('MyException') )
  {
     warn $e->error, "\n", $e->trace->as_string, "\n";
     warn join ' ',  $e->euid, $e->egid, $e->uid, $e->gid, $e->pid, $e->time;

     exit;
  }
  elsif ( $e = Exception::Class->caught('ExceptionWithFields') )
  {
     $e->quixotic ? do_something_wacky() : do_something_sane();
  }
  else
  {
     $e = Exception::Class->caught();
     ref $e ? $e->rethrow : die $e;
  }

  # use an alias - without parens subroutine name is checked at
  # compile time
  throw_fields error => "No strawberry", grandiosity => "quite a bit";

=head1 DESCRIPTION

Exception::Class allows you to declare exception hierarchies in your
modules in a "Java-esque" manner.

It features a simple interface allowing programmers to 'declare'
exception classes at compile time.  It also has a base exception
class, L<Exception::Class::Base>, that can be easily extended.

It is designed to make structured exception handling simpler and
better by encouraging people to use hierarchies of exceptions in their
applications, as opposed to a single catch-all exception class.

This module does not implement any try/catch syntax.  Please see the
"OTHER EXCEPTION MODULES (try/catch syntax)" section for more
information on how to get this syntax.

You will also want to look at the documentation for
L<Exception::Class::Base>, which is the default base class for all
exception objects created by this module.

=head1 DECLARING EXCEPTION CLASSES

Importing C<Exception::Class> allows you to automagically create
L<Exception::Class::Base> subclasses.  You can also create subclasses
via the traditional means of defining your own subclass with C<@ISA>.
These two methods may be easily combined, so that you could subclass
an exception class defined via the automagic import, if you desired
this.

The syntax for the magic declarations is as follows:

'MANDATORY CLASS NAME' => \%optional_hashref

The hashref may contain the following options:

=over 4

=item * isa

This is the class's parent class.  If this isn't provided then the
class name in C<$Exception::Class::BASE_EXC_CLASS> is assumed to be
the parent (see below).

This parameter lets you create arbitrarily deep class hierarchies.
This can be any other L<Exception::Class::Base> subclass in your
declaration I<or> a subclass loaded from a module.

To change the default exception class you will need to change the
value of C<$Exception::Class::BASE_EXC_CLASS> I<before> calling
C<import()>.  To do this simply do something like this:

BEGIN { $Exception::Class::BASE_EXC_CLASS = 'SomeExceptionClass'; }

If anyone can come up with a more elegant way to do this please let me
know.

CAVEAT: If you want to automagically subclass an
L<Exception::Class::Base> subclass loaded from a file, then you
I<must> compile the class (via use or require or some other magic)
I<before> you import C<Exception::Class> or you'll get a compile time
error.

=item * fields

This allows you to define additional attributes for your exception
class.  Any field you define can be passed to the C<throw()> or
C<new()> methods as additional parameters for the constructor.  In
addition, your exception object will have an accessor method for the
fields you define.

This parameter can be either a scalar (for a single field) or an array
reference if you need to define multiple fields.

Fields will be inherited by subclasses.

=item * alias

Specifying an alias causes this class to create a subroutine of the
specified name in the I<caller's> namespace.  Calling this subroutine
is equivalent to calling C<< <class>->throw(@_) >> for the given
exception class.

Besides convenience, using aliases also allows for additional compile
time checking.  If the alias is called I<without parentheses>, as in
C<throw_fields "an error occurred">, then Perl checks for the
existence of the C<throw_fields()> subroutine at compile time.  If
instead you do C<< ExceptionWithFields->throw(...) >>, then Perl
checks the class name at runtime, meaning that typos may sneak
through.

=item * description

Each exception class has a description method that returns a fixed
string.  This should describe the exception I<class> (as opposed to
any particular exception object).  This may be useful for debugging if
you start catching exceptions you weren't expecting (particularly if
someone forgot to document them) and you don't understand the error
messages.

=back

The C<Exception::Class> magic attempts to detect circular class
hierarchies and will die if it finds one.  It also detects missing
links in a chain, for example if you declare Bar to be a subclass of
Foo and never declare Foo.

=head1 Catching Exceptions

C<Exception::Class> provides some syntactic sugar for catching
exceptions in a safe manner:

 eval { ... }

 if ( my $e = Exception::Class->caught('My::Error') )
 {
     cleanup();
     do_something_with_exception($e);
 }

The C<caught()> method takes a class name and returns an exception
object if the last thrown exception is of the given class, or a
subclass of that class.  If it is not given any arguments, it simply
returns C<$@>.

You should B<always> make a copy of the exception object, rather than
using C<$@> directly.  This is necessary because if your C<cleanup()>
function uses C<eval>, or calls something which uses it, then C<$@> is
overwritten.  Copying the exception preserves it for the call to
C<do_something_with_exception()>.

Exception objects also provide a caught method so you can write:

 if ( my $e = My::Error->caught() )
 {
     cleanup();
     do_something_with_exception($e);
 }

=head2 Uncatchable Exceptions

Internally, the C<caught()> method will call C<isa()> on the exception
object.  You could make an exception "uncatchable" by overriding
C<isa()> in that class like this:

 package Exception::Uncatchable;

 sub isa { shift->rethrow }

Of course, this only works if you always call 
C<< Exception::Class->caught() >> after an C<eval>.

=head1 USAGE RECOMMENDATION

If you're creating a complex system that throws lots of different
types of exceptions, consider putting all the exception declarations
in one place.  For an app called Foo you might make a
C<Foo::Exceptions> module and use that in all your code.  This module
could just contain the code to make C<Exception::Class> do its
automagic class creation.  Doing this allows you to more easily see
what exceptions you have, and makes it easier to keep track of them.

This might look something like this:

  package Foo::Bar::Exceptions;

  use Exception::Class ( Foo::Bar::Exception::Senses =>
                        { description => 'sense-related exception' },

                         Foo::Bar::Exception::Smell =>
                         { isa => 'Foo::Bar::Exception::Senses',
                           fields => 'odor',
                           description => 'stinky!' },

                         Foo::Bar::Exception::Taste =>
                         { isa => 'Foo::Bar::Exception::Senses',
                           fields => [ 'taste', 'bitterness' ],
                           description => 'like, gag me with a spoon!' },

                         ... );

You may want to create a real module to subclass
L<Exception::Class::Base> as well, particularly if you want your
exceptions to have more methods.

=head2 Subclassing Exception::Class::Base

As part of your usage of C<Exception::Class>, you may want to create
your own base exception class which subclasses
L<Exception::Class::Base>.  You should feel free to subclass any of
the methods documented above.  For example, you may want to subclass
C<new()> to add additional information to your exception objects.

=head1 Exception::Class FUNCTIONS

The C<Exception::Class> method offers one function, C<Classes()>,
which is not exported.  This method returns a list of the classes that
have been created by calling the C<Exception::Class> import() method.
Note that this is I<all> the subclasses that have been created, so it
may include subclasses created by things like CPAN modules, etc.  Also
note that if you simply define a subclass via the normal Perl method
of setting C<@ISA> or C<use base>, then your subclass will not be
included.

=head1 OTHER EXCEPTION MODULES (try/catch syntax)

If you are interested in adding try/catch/finally syntactic sugar to
your code then I recommend you check out U. Arun Kumar's C<Error.pm>
module, which implements this syntax.  It also includes its own base
exception class, C<Error::Simple>.

If you would prefer to use the L<Exception::Class::Base> class
included with this module, you'll have to add this to your code
somewhere:

  push @Exception::Class::Base::ISA, 'Error'
      unless Exception::Class::Base->isa('Error');

It's a hack but apparently it works.

=head1 SUPPORT

Please submit bugs to the CPAN RT system at
http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Exception%3A%3AClass or
via email at bug-exception-class@rt.cpan.org.

=head1 DONATIONS

If you'd like to thank me for the work I've done on this module,
please consider making a "donation" to me via PayPal. I spend a lot of
free time creating free software, and would appreciate any support
you'd care to offer.

Please note that B<I am not suggesting that you must do this> in order
for me to continue working on this particular software. I will
continue to do so, inasmuch as I have in the past, for as long as it
interests me.

Similarly, a donation made in this way will probably not make me work
on this software much more, unless I get so many donations that I can
consider working on free software full time, which seems unlikely at
best.

To donate, log into PayPal and send money to autarch@urth.org or use
the button on this page:
L<http://www.urth.org/~autarch/fs-donation.html>

=head1 AUTHOR

Dave Rolsky, E<gt>autarch@urth.orgE<lt>

=head1 COPYRIGHT

Copyright (c) 2000-2009 David Rolsky.  All rights reserved.  This
program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.

The full text of the license can be found in the LICENSE file included
with this module.

=cut
